Various methods of spinal immobilization have been known and used during this century in the treatment of spinal instability and displacement. The preferred treatment for spinal stabilization is immobilization of the joint by surgical fusion, or arthrodesis. This method has been known since its development in 1911 by Hibbs and Albee. However, in many cases, and in particular, in cases involving fusion across the lumbosacral articulation and when there are many levels involved, pseudoarthrosis is a problem. It was discovered that immediate immobilization was necessary in order to allow a bony union to form. Early in the century, post operative external immobilization such as the use of splints and casts was the favored method of treatment, however, as surgical techniques have become more sophisticated, various methods of internal and external fixation have been developed.
Internal fixation refers to therapeutic methods of stabilization which are wholly internal to the patient and include commonly known devices such as bone plates and pins. External fixation in contrast involves at least some portion of the stabilization device which is external to the patient's body. Internal fixation is now the favored method of immobilization since the patient is allowed greater freedom with the elimination of the external portion of the device and the possibility of infections, such as pin tract infection, is reduced.
Some of the indications treated by internal fixation of the spine include vertebral displacement and management such as kyphosis, spondylolisthesis and rotation; segmental instability, such as disc degeneration and fracture caused by disease and trauma and congenital defects; and tumor diseases.
A common problem with spinal fixation is the question of how to secure the fixation device to the spine without damaging the spinal cord. The pedicles are a favored area of attachment since they offer an area that is strong enough to hold the fixation device even when the patient suffers from osteoporosis. Since the middle 1950's, methods of fixation have utilized the pedicles. In early methods, screws extended through the facets into the pedicles. More recently, posterior methods of fixation have been developed which utilize wires that extend through the spinal canal and hold a rod against the lamina (such as the Luque system) or that utilize pedicular screws which extend into the pedicle and secure a plate which extends across several vertebral segments (such as the Steffee plate).
U.S. Pat. No. 4,805,602 to Puno, et al. presents a system sharing advantage of both the wired implants and the plate. Specifically, that screw and rod system provides a rigidity which is intermediate between the wired implant and the plate systems and may be contoured to any plane.
The present invention represents an improvement in the technology and in the therapy advanced in U.S. Pat. No. 4,805,602. In particular, this invention greatly reduces the time required to perform the spinal operation as compared to the prior invention. As an example of such a reduction, the time for inserting the anchors may be cut from hours to around an hour. Such a time saving represents a significant reduction in the risk associated with a surgical procedure. Further, the new design may be easier to use as the chances of cross-threading the nut unto the anchor are reduced and the nut is more accessible for tightening. This is of particular significance in the bloody environment which obscures the spinal surgeon's access to the fixation device. The present device achieves this accessibility and attendant time savings without sacrificing the mechanical benefits of the earlier design. In particular, the anchor is designed so that it is not overly obtrusive. More specifically, the nut is thin and further is chamfered to reduce bulk and yet includes a thread design to achieve sufficient compression on the rod. The anchor system presents a flush upper surface and the total system is elegant and effective. Each anchor seat is secured by a cancellous screw which cooperates through a sloped bore in the anchor seat so as to provide a limited ball and socket motion. The design of the present invention incorporates a method of therapy for treating a spinal indication utilizing this internal fixator.
In particular, the present invention is viewed as having an application in the stabilization of the thoracolumbar, lumbar, and sacral spine. There are problems of fixation unique to this area of the spine such as the fact that the lumbar spine is normally lordotic and this lordosis must be preserved. In addition, indicated spinal decompression often requires a destabilization of the spine posteriorly. This may result in instability unless fusion is done, and fusion will often fail to become solid unless effective internal fixation is used. Finally, the points of sacral fixation are the weakest point of fixation. These problems are addressed by the present invention.